In a rechargeable battery pack, a multiplicity of individual cells are regularly interconnected in series and/or in parallel to provide a rechargeable battery pack having a desired supply voltage and capacity. In the new state, the individual cells have substantially an identical behavior and identical capacity; with increasing aging of the cells as a result of temperature influences, storage and loading, the performance of the individual cells will deviate from one another. Since a cell network, in particular a series circuit having individual cells, is in principle only as good as the weakest cell, measures are provided to recognize, at an early stage, that the rechargeable battery pack can no longer provide the required power levels and must be exchanged.
It is known, for monitoring a cell network composed of a multiplicity of individual cells, to feed each individual cell voltage to a monitoring circuit to detect the magnitude of each individual cell voltage. The detected voltages of the individual cells are then compared among one another in order to ascertain the maximum voltage difference between a weakest cell and a strongest cell. This voltage difference is a variable for the unbalancing of a cell network in a rechargeable battery pack; if the voltage difference is small, the individual cells of the cell network are in a good state; if the voltage difference becomes large, individual cells are significantly weaker than stronger cells, and the rechargeable battery pack is usable only to a limited extent. If the difference voltage exceeds predefined unbalancing limits, the rechargeable battery pack is switched off.
In the new state of a cell network, for precise monitoring of the unbalancing of a rechargeable battery pack, the unbalancing limit can be set narrowly; as the cell network ages, the unbalancing increases, but the rechargeable battery pack can still be used. It is only when predefined unbalancing limits are exceeded that the rechargeable battery pack has to be deactivated.
The unbalancing limit, of a cell network in a rechargeable battery pack is designed according to what maximum permissible unbalancing can still be permitted in the aging state of the individual cells; in the new state of the rechargeable battery pack, accurate monitoring is not possible on account of the large unbalancing limits. Therefore, an incipient failure of an individual cell already in the new state of a rechargeable battery pack cannot be identified.